The present invention relates to an original reading device for reading image information on an original. More particularly, the invention pertains to a original reading device having an automatic document feeding function.
One known method of reading an original used in an original reading device is an original moving method in which image information on the original is read as the original is fed through an original transport path. The original moving method, widely used in the original reading device in recent years, has an advantage that a plurality of originals loaded on a document tray can be automatically transported to an original reading position and read one after another.
The original moving method, however, has a problem that if unevenness in original transport speed occurs at the original reading position, this unevenness in the original transport speed results in irregularities in a read image. It is therefore important for the original reading device employing the original moving method to ensure that the original transport speed does not become uneven at the original reading position.
One example of a prior art arrangement for the solution of this problem is shown in Japanese Laid-open Patent Application No. H04-7237. A document feeder described in this Patent Application includes a transport roller located tight against an original reading portion with an original transport path formed between the original reading portion and the transport roller. The transport roller feeds an original downstream along the original transport path while pressing the original against the original reading portion at the original reading position.
The document feeder of Japanese Laid-open Patent Application No. H04-7237 includes a pair of rotary motion transmission mechanisms provided at both axial ends of the transport roller, each rotary motion transmission mechanism including an untoothed pulley and an untoothed belt. Since the untoothed belt of each rotary motion transmission mechanism properly slips over the untoothed pulley, the rotary motion transmission mechanisms located at both axial ends of the transport roller are not likely to produce a phase lag. For this reason, it is generally appreciated that the document feeder of this Patent Application is unlikely to produce unevenness in original transport speed.
However, the aforementioned arrangement of the document feeder of Japanese Laid-open Patent Application No. H04-7237 does not take into consideration unevenness in the original transport speed occurring at the original reading position due to a relation between the transport roller and an original feed roller. A main factor that causes this unevenness in the original transport speed is a load fluctuation which occurs when a trailing edge of the original passes beyond the original feed roller.
Since this load fluctuation is not taken into consideration in the design of conventional document feeders, there is a risk of degradation of original reading accuracy due to the unevenness in the original transport speed at the original reading position.